Method of producing dye-sensitized solar cell and an electrode of a dye-sensitized solar cell

ABSTRACT

A method of producing an electrode of a dye-sensitized solar cell includes dispersing semiconductor nanoparticles on a transparent electrically conductive substrate, dispersing semiconductor nanofibers on the semiconductor nanoparticle layer, adsorbing onto all sides of the semiconductor nanofibers a first light absorption material, thereby sensitizing the semiconductor nanofibers, wherein the light absorption material has a first light absorption bandwidth, and depositing a second light absorption material in contact with and forming respective shells on the respective semiconductor nanofibers on which the first light absorption material is adsorbed, wherein the second light absorption material has a second light absorption bandwidth complementary to the first light absorption bandwidth.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a dye-sensitized solar cell thatconverts solar energy to electric energy, and more particularly, todye-sensitized solar cells that include ultra-fine semiconductor fiberssensitized with more than one light absorption materials.

Description of the Related Art

Dye-sensitized solar cell (DSSC) is a non-conventional photovoltaictechnology that attracted much attention due to its cost-effectivenessin harvesting solar energy with appealing properties such asflexibility, transparency, and adaptability in large-area devices. Theoperating principle of DSSC is illustrated in FIG. 12. Uponillumination, the dyes adsorbed onto the metal oxide semiconductor(usually TiO₂) are sensitized to the exited state (S*) by lightabsorption right at the interface and they dissociate readily to createan electron-hole pair, with electrons subsequently injected into theconduction band of the semiconductor while the holes, at leastinitially, remain on the sensitizers. The dye ground state (S) is thenregenerated by electron donation from the redox system to the oxidizedstate of the sensitizer (S⁺). The recuperation of redox system isrealized by transporting holes to the counter electrode either indiffusion or hopping mechanism (depending on the transporting mediator).The whole process is finally completed by electron migration via theouter circuit and the device generates electric power from light withoutchemical transformation.

For decades, DSSC have become one of the most efficient and stableexcitonic solar cells. A central feature of this device is utilizingphotosensitizing dye that harvests light and generates excitons. Inorder to achieve high power conversion efficiency based on I⁻/I₃ ⁻ redoxcouple system which would be competitive with conventional silicon solarcells, DSSC must absorb as much as 80% of solar spectrum with wavelength between 350 and 900 nm. While the traditional ruthenium-baseddyes exhibit relative broad adsorption spectrum, it has difficulty infurther improving its Power Conversion Efficiency (PCE) due to its lowmolar extinction coefficients.

Organic dyes, such as metallophthalocyanines (MPcs), shows higher molarextinction coefficient (100,000 M⁻¹cm⁻¹), however, they have narrowabsorption bandwidth. Complementally, dye cocktails or co-sensitizationhas been proposed to enhance the light absorption and extend theabsorption spectrum. However, it has achieved only limited successto-date. This is probably due to (i) inferior injection efficiencycaused by intermolecular interactions between dyes; (ii) confinedsurface areas of the photoanode for dyes to be absorbed. Considerableefforts have been made to solve these problems, one option is toseparate the adsorption sites on TiO₂, which means achieving the properposition of each dye on the desired site, however, there is difficultyin realizing such a concept.

Recently, there have been some efforts on the use of Frster resonanceenergy transfer (FRET) in DSSC to enhance the light harvesting where anunattached, highly luminescent donor dye was inside the electrolyte toabsorb high energy photons and efficiently transfer the energy to theanchored near-infrared acceptor dye. Unfortunately, I₃ ⁻ in theelectrolyte was found to partially quench the fluorescence of thedonors, therefore only limited improvement in device performance can beachieved with such approach.

In view of the deficiencies of the conventional dye-sensitized solarcells, there is an increasing demand for high efficiency solar cellsthat are capable of harvesting a broader range of solar energy withimproved power conversion efficiency.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a dye-sensitizedsolar cell that includes an electrode having a semiconductornanoparticle layer dispersed on a transparent conductive substrate, aplurality of semiconductor nanofibers dispersed on the nanoparticlelayer, a first light absorption material is attached to the plurality ofsemiconductor nanofibers in which the first light absorption materialhaving a first light absorption bandwidth, and a second light absorptionmaterial deposited on the first light absorption material of theplurality of semiconductor nanofibers, the second light absorptionmaterial having a second light absorption bandwidth complementary to thefirst light absorption bandwidth, a counter electrode includes ametal-coated transparent conductive substrate, and an electrolyte incontact with the second light absorption material and the counterelectrode.

According to another aspect, the present invention provides a method ofproducing an electrode of a dye-sensitized solar cell, the methodincludes preparing a transparent conductive substrate, dispersing aplurality of semiconductor nanoparticles on the transparent conductivesubstrate, dispersing a plurality of semiconductor nanofibers on thesemiconductor nanoparticle layer, sensitizing the semiconductornanofibers with a first light absorption material in which the lightabsorption material having a first light absorption bandwidth, anddepositing a second light absorption material on the first lightabsorption material, the second light absorption material having asecond light absorption bandwidth complementary to the first lightabsorption bandwidth.

According to yet another aspect, the present invention provides a methodof producing an electrode of a dye-sensitized solar cell, the methodincludes dispersing a plurality of semiconductor nanoparticles,dispersing a plurality of semiconductor nanofibers on the semiconductornanoparticle layer, sensitizing the semiconductor nanofibers with afirst light absorption material in which the light absorption materialhaving a first light absorption bandwidth and an energy level higherthan that of a conduction band of the semiconductor nanofibers, anddepositing more than one light absorption materials, successively, inform of a shell structure on top of a preceding light absorptionmaterial, with each successive light absorption material having a higherenergy level than the light absorption material in the preceding lightabsorption material.

Further features and aspects of the present invention will becomeapparent from the following description of exemplary embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 illustrates a schematic of a semiconductor nanofiberdye-sensitized solar cell (DSSC) in accordance with one embodiment ofthe present invention.

FIG. 2a schematically illustrates a recombination process in DSSCwithout CuPc deposition.

FIG. 2b schematically illustrates a recombination process in DSSC withCuPc deposition.

FIG. 3a illustrates the holes transfer process in DSSC with N719/CuPcsensitized TiO₂ nanofibers photoanode with a thinner CuPc shell.

FIG. 3b illustrates the holes transfer process in DSSC with N719/CuPcsensitized TiO₂ nanofibers photoanode with a thicker CuPc shell.

FIG. 4 illustrates a schematic of electrolyte quenching the excitedstate of the CuPc, and relationship between exciton concentration anddistance from closer to N719 to away from N719.

FIG. 5 illustrates a graph of photocurrent density vs. voltage (J-V)characteristic of device with a photoanode of CuPc/TiO₂.

FIG. 6a illustrates TiO₂ nanofibers without CuPc coating; FIGS. 6b, 6c,and 6d illustrates N719 sensitized TiO₂ nanofiber having a CuPcdeposition thickness of 20 nm, 30 nm, and 40 nm, respectively.

FIG. 7a illustrates scanning electron microscope (SEM) images of TiO₂nanofiber sensitized with N719 before the coating of a CuPc layer.

FIG. 7b illustrates a SEM image of TiO₂ nanofiber after a CuPc layer isdeposited on the sensitized TiO₂ nanofibers in a core-shell structure.

FIG. 7c illustrates a Transmission Electron Microscopy (TEM) image ofTiO₂ nanofiber after a CuPc layer is coated on the sensitized TiO₂nanofibers in a shell-like structure.

FIG. 8a illustrates absorption spectrums of photoanode with N719/TiO₂,CuPc/TiO₂, CuPc/N719/TiO₂ structures.

FIG. 8b illustrates emission spectrum of CuPc and N719/TiO₂.

FIG. 9 illustrates photocurrent density-voltage (J-V) characteristics ofDSSC devices with and without a CuPc layer.

FIG. 10a illustrates EQE versus wavelength of the DSSC with and withoutCuPc (30 nm); and FIG. 10b illustrates EQE addition (AEQE) caused by 30nm CuPc and corresponding absorption spectrum of CuPc on TiO₂.

FIG. 11 illustrates photovoltage characteristic of DSSC devices withvarious thickness of CuPc; FIG. 11a is a graph that illustrates PCEversus various thickness of CuPc; FIG. 11b is a graph that illustratesJ_(sc) versus various thickness of CuPc; FIG. 11c is a graph thatillustrates V_(oc) versus various thickness of CuPc; FIG. 11d is a graphthat illustrates FF versus various thickness of CuPc.

FIG. 12 illustrates the general principle of operation of ananocrystalline TiO₂ dye-sensitized solar cell.

DESCRIPTION OF THE EMBODIMENTS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

DSSC offers the potential of low-cost, high-efficiency photovoltaicdevice. However, conventional DSSC cannot utilize all of the photons ofthe visible solar spectrum and consequently the realized powerconversion efficiency (PCE) is limited. The present invention disclosesa core-shell photoanode, where a thin shell of infrared dye is depositedon the core of sensitized TiO₂ nanofiber. In such architecture, photonsare absorbed by the infrared dye and undergo charge transfer to thesensitizing dye, which broadens the absorption spectrum and suppressesthe recombination process (electron back reaction and recombination withelectrolyte ions). In one embodiment, ruthenium-based dye (N719)sensitized TiO₂ nanofibers are wrapped by thin-shell of copperphthalocyanine (CuPc) achieving a high efficiency of 9.48%. Rather thanthe typical Frster resonance energy transfer (FRET), the charge transferbetween the CuPc and N719 molecules involves organized energy levelswhich can be explained by the microscopic more efficient indirectelectron transfer process. Such an innovative approach provides analternative method for enhancing the performance of DSSC for low-costrenewable energy generation in the future.

FIG. 1 illustrates a schematic of a semiconductor nanofiberdye-sensitized solar cell (DSSC) in accordance with one embodiment ofthe present invention. Referring to FIG. 1, the DSSC 100 includes asemiconductor electrode (photoanode) 180 and a counter electrode 110separated by electrolyte 120, which can be a liquid or a gel. Thesemiconductor electrode 180 comprises a transparent electricallyconductive substrate (e.g. fluorine-doped tin dioxide (FTO) glasssubstrate), 170, a semiconductor nanoparticle layer 160, and a nanofiberlayer 130. The counter electrode 110 can be produced with a variety ofelectrically conductive materials.

In one embodiment, the semiconductor nanofiber layer 130 is made withtitanium dioxide (TiO₂) nanofibers which are adsorbed with a first lightabsorption material such as ruthenium-based dye molecules (e.g., N719,N709, N3, C101, etc.), and further deposited with a second lightabsorption material such as copper phthalocyanine (CuPc), zincphthalocyanine (ZnPc), phthalocyanine sensitizers and other organic dyes(e.g. porphyrin sensitizers). The second light absorption materialhaving a second light absorption bandwidth complementary to the lightbandwidth of the first light absorption material.

The photoanode 180 and the counter electrode 110 are separated with amediator such as electrolyte 120, which can take form of a liquid or agel. In the latter case, an appropriate gelator such as poly (ethyleneglycol), poly (methyl methacrylate), etc. is further added to gelate theelectrolyte solution. The electrolyte may include iodide/triiodide, orothers such as cobalt in form of Co(II)/(III), redox couple that isadapted to transport electrons from the counter electrode to thenanofiber layer to replenish the sensitized dyes. The counter electrodecan be a platinum-sputtered FTO glass or an indium tin oxide (ITO) glasssubstrate. The counter electrode and dyes anchored TiO₂ photoanode isassembled into a sandwich structure with hot-melt film (Surlyn, DuPont,25 micro m).

The composite electrode materials are composed of semiconductornanofibers such as titanium dioxide nanofibers dispersed in a matrixwhich is coated with ruthenium-based dye molecules. Upon illumination,the ruthenium-based dye molecules are excited and created anelectron-hole pair. Subsequently, the electrons are injected into theconduction band (CB) of TiO₂ photoanode. The average diameter ofnanofibers is between 20-2,000 nm, with a preferred range of 30-100 nm.For the purpose of this disclosure, the diameter of a nanofiber refersto its cross-sectional diameter. As the cross-sectional of the nanofibermay not be circular, the equivalent diameter may equal to the averageperimeter of the non-circular fiber divided by 3.14159.

For illustrative purposes, in the examples below, the term “dye” and“light absorption material” are used in a broad sense can be usedinterchangeably. For illustrative purposes, the examples below may useN719 (i.e., first light absorption material) and CuPc (i.e., secondlight absorption material) to describe light absorption material andnear-infrared light absorption material, respectively. Thus, theexamples should not be construed as limiting the scope of the presentinvention.

According to one embodiment, an ultrathin layer of CuPc is deposited ontop of N719 sensitized TiO₂ nanofibers. The CuPc is capable ofharvesting complementary light energy and creating electron-hole pairs,and subsequently, injecting the electrons to N719. As a result, itbroadens the absorption bandwidth of DSSC which nearly covered theentire visible solar radiation spectrum.

There are two routes for charge generation and transfer in this system,which is illustrated in FIG. 1. Upon illumination, molecule of the lightabsorption material is excited and create an electron-hole pair,subsequently electron injects into the conduction band (CB) of TiO₂photoanode (path 102). In the present DSSC design, the CuPc layercomplementally absorbs near-infrared photon and creates an electron-holepair, exciton. The lowest unoccupied molecular orbital (LUMO) energylevel for CuPc is −3.5 eV. The exciton diffuses to the interface betweenCuPc and N719 molecules and dissociates due to the LUMO energy levelwhich is offset at the interface. The electrons transfer to the LUMO ofN719 initially (path 101) as the LUMO energy level for N719 is lower at−3.85 eV and subsequently inject into the CB of TiO₂ (at even lowerenergy level of −4.2 eV) via an indirect, more-efficient, chargetransfer process. It is to be noted that the excitons is diffused ortransported towards lower LUMO energy levels, and the light absorptionmaterial (in this case N719 with LUMO of −3.85 eV) is selected such thatits energy level should be higher than the CB of the semiconductornanofibers (−4.2 eV), while the near-infrared absorption materialsshould have LUMO energy level (in this case CuPc with LUMO of −3.5 eV)higher than the light absorption material for this indirect chargetransfer process to take effect. If CuPc is replaced by anothernear-infrared light absorption material such as ZnPc with LUMO of −3.34eV, this will also work as its LUMO is higher than N719 at −3.85 eV. Tothis end, the present invention relies critically on selecting the twolight absorption materials according to their energy levels in respectto each other as well as to the CB of the semiconductor nanofiber.

In another embodiment, more than one light absorption material can bedeposited on the semiconductor nanofibers, such as TiO₂ nanofibers withconduction band energy level of −4.2 eV. For example, N719 (with energylevel of −3.85 eV which is higher than −4.2 eV corresponding toconduction band of TiO₂) sensitized TiO₂ nanofibers are deposited with asecond light absorption material such as CuPc with LUMO energy level of−3.5 eV. Then, likewise, the second light absorption material isdeposited with a third light absorption material having an even higherenergy level, such as ZnPc with LUMO energy level of −3.34 eV, and soon. As such, each successive light absorption material forms ashell-like structure on the preceding light absorption material.Similarly, additional light absorption materials can be deposited witheach successive light absorption material having a higher energy levelthan the light absorption material of the preceding adjacent lightabsorption material. Each of the successive light absorption materialmay have a corresponding light absorption bandwidth that iscomplementary to the light absorption bandwidth of the preceding lightabsorption material.

The present photoanode design has a number of advantages. First, theone-dimensional nano-materials (e.g. TiO₂ nanofibers) maintain the fastcharge transport property. Second, the surface roughness of the fibers(polycrystallites less than 10 nm) is adapted for deposition of thelight absorption material. Third, the high pore volume or porosity ofthe photoanode as composed by nanofibers (in contrast with less porousphotoanode made up of nanoparticles) allows the second dye (CuPc) to bedeposited deeply into the photoanode. Suitable technique for depositingthe second dye include deep penetration method such as thermal,chemical, or physical vapor deposition which can permeate deeply in aporous structure, coating the light absorption materials (N719) of thesemiconductor nanofibers (TiO₂). Fourth, the near-infrared lightabsorption material (e.g. CuPc) deposited in form of an ultrathin layerexternal to the N719 sensitized TiO₂ nanofiber photoanode can expand theabsorption bandwidth while avoiding the competition of anchored sites onTiO₂ with the light absorption material. The latter has been a limitingfactor heretofore with conventional co-sensitization. Fifth, two typesof light absorption materials, or dyes, are spatially and energeticallyorganized so as to achieve electrons transfer from near-infrared (outerlayer) light absorption material to the first (inner layer) lightabsorption material while holes transfer from first light absorptionmaterial to the near-infrared light absorption material. This increasesthe distance between injected electrons and oxidized dye species (holes)thereby reduces, if not entirely suppresses, the recombination processbetween electrons and holes in the device.

For the DSSC with non-solid (i.e. liquid and gel) electrolytes,recombination occurs between the electrons in the conduction band (CB)of TiO₂ with the holes in the oxidized dye as shown in FIG. 2a .Referring to FIG. 2b , the dye N719 and CuPc are spatially andenergetically organized so as to achieve electron transfer from CuPc toN719 while holes transfer from N719 to CuPc. Consequently, the distancebetween injected electrons and holes is increased and therebysuppressing recombination process in the device.

FIGS. 3a and 3b schematically illustrate two different configurations ofthe present invention. FIG. 3a illustrates a holes transfer process inDSSC with N719/CuPc sensitized TiO₂ nanofibers photoanode with a thinnerCuPc shell; FIG. 3b illustrates a holes transfer process in DSSC withN719/CuPc sensitized TiO₂ nanofibers photoanode with a thicker CuPcshell. When thickness of CuPc is less than or equal to the “sum” or“total” of the exciton diffusion length (i.e., length or distance of thediffusion region) and the thickness of excitons quenched region, theexcitons diffuse to the interface of N719 and CuPc and subsequentlydissociate. The electrons are injected into LUMO of N719 and aresubsequently injected into CB of TiO₂. The holes can efficiently diffuseto the interface between CuPc and electrolyte. However, when thethickness of CuPc shell exceeds the “sum” or “total” of the length ofexciton diffusion and the thickness of quench region (FIG. 3b ), theexcess CuPc induces extra resistance that retards the regeneration ofthe N719 due to the inferior holes transport ability.

FIG. 4 is a schematic of electrolyte (I₃ ⁻) quenching the excited stateof the CuPc, and relationship between exciton concentration withdistance from closer to N719, to away from N719.

As is known, I₃ ⁻ in electrolyte is a “perfect quencher”, which iscapable of quenching the excited state of CuPc thereby the outmost shellof CuPc, which is being quenched, becomes malfunction or sacrificed. Assuch, an appropriate shell thickness should be optimally equal to thesum of the exciton diffusion length (depending on thecrystalline/non-crystalline structure of the near-infrared lightabsorption material) and the quench region thickness. The left region(401) is the exciton diffusion region, while the right region (403) is aquench region.

FIG. 5 illustrates a graph of photocurrent density vs. voltage (J-V)characteristic of device with the photoanode of CuPc/TiO₂ under AM1.5(100 mWcm⁻²). It shows the realized current density J for a givenvoltage V with J_(sc) corresponding to the short-circuit current densityat zero voltage and V_(oc) corresponding to the open-circuit voltagewithout current density. The maximum power from the DSSC devicecorresponds to a point (V, J) on the performance curve, whereby thepower, which is a product of J and V becomes maximum. The fill factor,FF, expressed as a percent, is defined as the ratio of the maximum powerto the product of theoretical power realized J_(sc)V_(oc). Higher FFrepresents better maximum power obtained from the DSSC device.

To determine the appropriate thickness of CuPc in the present device,photoanode deposited with different thickness of CuPc were tested in therange of 20 nm to 40 nm. The SEM images and respective photovoltaicproperties (V_(oc), J_(sc), FF and PCE) of these photoanodes were shownin FIGS. 6a-6d , FIGS. 7a-7c , and FIGS. 11a-11d . Referring to FIG. 11,V_(oc) of all devices almost remained as a constant of 0.73±0.02 V. FFwas maintained at 60% when thickness of CuPc layer was no more than 30nm, thereafter FF decreased to 54% when the CuPc thickness increased to40 nm. The highest efficiency of 9.48% with the highest J can beachieved when the CuPc layer was 30 nm. Experimental results show thatincreasing, or decreasing, the thickness of CuPc would lead to lowerperformance of the DSSC device. There was an optimum device design formaximum J and PCE. This is in agreement with what has been stated thatthe total CuPc thickness should be optimally equals to the sum of thediffusion length of the excitons in the CuPc and the thickness of thequench region. With increasing thickness of CuPc in the photoanode, moreelectrons are generated under solar irradiation and they get transferredto the TiO₂ photoanode via the N719, resulting in improved performance.However, when the thickness of CuPc exceeds the sum of the quench-layerthickness and the exciton diffusion length, the excess CuPc incurs extraresistance for holes transport (see FIG. 11) with consequence ofretarding the regeneration of N719 by the electrons. The thickness ofCuPc layer should be at least 5 nm with a preferred range of 10 and 50nm. For a particular embodiment as shown in FIG. 11a -FIG. 11b , thethickness of CuPc shell gives maximum of PCE and J_(sc).

FIGS. 6b-6d show SEM images of CuPc coated N719 sensitized TiO₂nanofiber with various thickness of CuPc deposition. More specifically,FIG. 6a illustrates TiO₂ nanofibers without CuPc coating; FIGS. 6b, 6c,and 6d illustrates N719 sensitized TiO₂ nanofiber having a CuPcdeposition thickness of 20 nm, 30 nm, and 40 nm, respectively.

FIG. 7a illustrates scanning electron microscope (SEM) images of TiO₂nanofiber sensitized with N719 before the coating of a CuPc layer. FIG.7b illustrates a SEM image of TiO₂ nanofiber after a CuPc layer isdeposited on the sensitized TiO₂ nanofibers in a core-shell structure.FIG. 7c illustrates a Transmission Electron Microscopy (TEM) image ofTiO₂ nanofiber revealing the core-shell structure after a CuPc layer iscoated on the sensitized TiO₂ nanofibers. Due to the high pore-volumeand high permeability of the nanofiber film layer together withtransporting CuPc in form of vapor (produced by thermal, chemical, orphysical vapor deposition) deeply into the nanofiber film layer, CuPc isable to deposit on the sensitized nanofibers to form core-shellstructures as shown in FIGS. 7b and 7c . The core-shell design is alsoable to improve and facilitate light harvesting, while reducing loss inenergy conversion from electron-hole recombination.

Next, an exemplary fabrication process of a photoanode in accordancewith one embodiment of the present invention is explained. To begin thefabrication process of the semiconductor nanofiber photoanode, a pieceof FTO glass is prepared. Next, a first layer of TiO₂/PVP compositenanofibers are electrospun on the FTO glass from a precursor solutionwhich contains titanium isopropoxide (TIP, 1.2 g), polyvinyipyrrolidone(PVP, 1 g), acetic acid (1 g) and ethanol (30 mL) Other materials withgood affinity to titanium dioxide such as polyvinylacetate,polyvinylalcohol, polyethyleneoxide and the like may also be used toprepare the precursor solution. Besides the electrospinning methoddescribed above, the semiconductor nanofibers can be produced by otherknown techniques such as chemistry based solution method. In someembodiments, the average TiO2 nanofiber length in a distribution of TiO2nanofibers ranges from nanometer scale to micro or millimeter scale. Asimilar nanofibers fabrication process is disclosed in U.S. patentapplication Ser. No. 13/244,957, entitled “Bilayer Dye-Sensitized SolarCell and Fabrication Method Thereof”, the information thereof isincorporated by reference herein in its entirety.

The diameter of the electrospun nanofibers can be influenced byprocessing parameters, and the diameter of the semiconductor (TiO₂)nanofibers can be controlled by adjusting the discharge amount, appliedvoltage for electrospinning, distance between positive electrode andground, and the consistency of the electrospinning solution. Thethicknesses of the nanofiber layer can be controlled by theelectrospinning time.

Thereafter, a calcination step is performed on the nanofibers in 450° C.for 2 hours. After calcination, another piece of FTO glass is preparedand a thin layer of TiO₂ nanoparticles is formed on the FTO glass bydoctor blading technique. Thereafter, the pilled semiconductor nanofiberfilm is placed on top of the nanoparticles layer in which thenanoparticles layer serves as a hole-blocking layer as well as a bondinglayer. Subsequently, this photoanode is calcinated at 450° C. for 2hours. The calcinated photoanode is further treated with an aqueoussolution of TiCl₄ (40 mM) at 60° C. for 15 min. Next, the obtained TiO₂nanofibers photoanode is sensitized in a solution of 0.03 mM Ru dye(N719) solution in absolute ethanol at 50° C. for 24 hours. The soakedphotoanode is then washed with ethanol to remove unanchored dyemolecules and then the photoanode is left dried. Thereafter, CuPc isdeposited via vapor deposition method, which can be thermal, chemical orphysical. The high “mobility” of the vapor of the infrared lightabsorption material can permeate/penetrate deeply into the photoanode,thereby depositing a shell-like structure onto the sensitized nanofibersin the photoanode. Otherwise, the deposition of the near-infrared dyemight only take form of a thin layer or coating on the surface orperiphery of the photoanode should the photoanode, made up ofnanoparticles, be of low-permeability and low porosity. (In the latter,the core-shell structure will not be realized, and there would bedifficulty to harness the benefits of more efficient indirect chargetransfer as well as the reduced recombination.) The deposition methodwill be explained in more details below in connection with theexperimental setup.

FIG. 8a illustrates an experimental result of the absorption spectrum ofsensitized photoanodes having N719/TiO₂, CuPc/TiO₂ and CuPc/N719/TiO₂structure, respectively. The absorption bandwidth of a single N719sensitized TiO₂ was in the range of 400 nm to 550 nm. After inducing theshell layer, CuPc, the absorption spectrum can be broadened up to thenear-infrared range. Furthermore, the photoluminescence (PL) emissionspectrums of CuPc, N719/TiO₂ and CuPc/N719/TiO₂ photoanodes are shown inFIG. 8b which exhibits no evident emission occurring in the wavelengthof 330 to 800 nm for CuPc. On the other hand, the emission intensity ofN719/TiO₂ was enhanced with introduction of CuPc, which suggests photonswere absorbed by CuPc and subsequently transferred to N719, resulting inenhanced PL emission. Given there is no overlap between the absorptionspectrum of N719 and the emission spectrum of CuPc, the FRET process,which is highly dependent on the overlap, according to Frster theory,does not work efficiently in the present N719-sensitized TiO₂ core—CuPCshell device.

FIG. 9 shows an experimental result of photocurrent density-voltage(J-V) characteristics of DSSC with and without CuPc layer (30 nm) underAM1.5 (100 mWcm⁻²) illumination, and related data is shown in Table 1below:

TABLE 1 Photovoltaic characteristics of DSSC with and without CuPc 30 nmCuPc Without CuPc Change % J_(sc) (mAcm⁻²) 21.12 14.97 41.08 V_(oc) (V)0.74 0.75 −1.33 FF (%) 60.65 56.91 6.57 PCE (%) 9.48 6.39 48.36 R_(s)(Ωcm²) 2.34 10.07 −76.76 R_(p) (Ωcm²) 641.5 414.8 54.65The thickness of photoanode (excluding FTO glass) in these devices wasmaintained at 13±1 μm. The open circuit voltage (V_(oc)), at 0.74-0.75V, remained nearly unchanged for DSSC with or without CuPc deposition(see in Table 1). The device sensitized with only N719 has a PCE of6.39% while DSSC with a 30 nm CuPc shell demonstrated an enhancement upto 9.48% PCE, a 48% increase in device performance. The improvementprimarily attributed to the increase in both short-circuits currentdensity (J) and fill factor (FF). However, for reference the device withphotoanode of only CuPc/TiO₂ was found to have extremely low J and PCE,respectively, 0.67 mAcm⁻² and 0.18%, as shown in FIG. 5. It is knownthat CuPc with multiple and symmetrical carboxyl groups is easy tosynthesize however sensitizing TiO₂ is difficult as the electron cannotdirect inject from CuPc into the CB of TiO₂ due to the wide band gap.Thus, the device with CuPc/TiO₂ photoanode shows extremely low PCE. Theimproved performance as evident from the very high J realized throughintroducing the CuPc shell was due to the indirect charge transfer fromthe CuPc, through N719, to the CB of TiO₂. Therefore, there were tworoutes for charge generation and transport incorporated in the presentnovel system. Besides harvesting light, N719 also functions as a chargecarrier or transport channel connecting CuPc and TiO₂.

The 48% enhancement in device performance is mainly due to increase ofJ_(sc) which can be examined by an increase in the external quantumefficiency (EQE) in the two wave-length regimes: 350 to 400 nm and 550to 800 nm as depicted in FIG. 10a . The increase of EQE is attributed tothe introduced CuPc that can absorb photons in these regions foradditional charge generation. As shown in FIG. 10b , the ECEenhancement, ΔEQE, was calculated based on the difference between EQE ofthe device with and without CuPc. The ΔEQE has three peaks,respectively, at 355, 570 and 700 nm, corresponding to 28%, 15%, and 27%enhancement in EQE that matches the absorption spectrum of pure CuPcvery well. The foregoing analysis clearly reveals the effect of CuPc onperformance of the DSSC device.

The experimental data demonstrate the viability of application of thepresent photoanode with core-shell structure in DSSC. The core (TiO₂)sensitized with a dye (i.e., first light absorption material) and theshell made of another dye (i.e., second light absorption material) hasthe complementary absorption region thus could broaden the absorptionspectrum of device. Furthermore, the shell can suppress recombinationprocess in the device due to the organized level energy between twodyes. Realization of high efficiency device in excess 15%, a core-shellphotoanode with much stronger dye instead of CuPc would be a more viablesolution for deploying this type of DSSC device for renewable energy inthe future.

Experimental Setup

Preparation of the Novel Core-Shell Photoanode:

A TiO₂ nanofiber photoanode (3 mm×3 mm) with the thickness of (13±1) μmis prepared. The prepared TiO₂ nanofiber film was first treated with anaqueous solution of TiCl₄ (40 mM) at 60° C. for 15 min. After treatment,washing with ethanol and drying in vacuum at 80° C., and immersed in asolution of 0.03 mM N719 in absolute ethanol at 55° C. for 24 hours.Prior to loading into vacuum chamber, the soaked photoanode was washedwith ethanol to remove “unanchored” dye. A CuPc layer was deposited onN719 sensitized TiO₂ nanofiber photoanode using thermal evaporationunder a pressure of <10⁻⁶ Torr at a deposition of around 0.5 Å/s, whichwas rotated at a rate during deposition. A 6 MHz gold crystal monitorwas used to determine film thickness and deposition rate.

The morphology of the photoanode was investigated by images obtainedfrom scanning electron microscopy (SEM, Hitachi S4800) and transmissionelectron microscope (TEM, JEOL 2100F). The core-shell TEM samples wereprepared by directly filing the CuPc/N719/TiO₂ photoanode bygraphite-covered copper grid.

The absorption spectrum of N719/TiO₂, CuPc/TiO₂ and CuPc/N719/TiO₂photoanodes was measured by an Agilent Varian Cary 4000 UV/VIS/NIRspectrophotometer. Photoluminescence (PL) data was measured at roomtemperature using an Edinburgh FLSP920 spectrophotometer with anincidence-and-detection angle of 45°. The emission spectrum was measuredat an excitation using a 32-nm monochromatic filter and an increment of1 nm was adopted for data collection.

Solar Cell Fabrication and Characterization:

Platinum-sputtered FTO glass was used as the counter electrode. Thecounter electrode and dyes anchored TiO₂ photoanode were assembled intoa sandwich prototype with hot-melt film (Surlyn, DuPont, 25 μm). Theinternal space of the cell was filled with a liquid electrolyte, whichconsisted of a mixture of 0.6 M 1-methy-3-propylimidazolium iodide(PMII), 0.05 M LiI, 0.05 M I₂, and 0.5 M 4-tert-butyl pyridine (TBP) inacetonitrile. The prepared DSSC device was subject to illumination ofAM1.5G 100 mWcm⁻² from a solar simulator ABET SUN 2000 with powerdensity calibrated by a silicon reference cell (NIST). The performanceof the DSSC device was monitored by a power meter (Keithley 2400 digitalsource meter) throughout the testing. The external quantum efficiency(EQE) values were measured with an EQE system equipped with a xenon lamp(Oriel 66902, 300 W), a monochrometor (Newport 66902), a Si detector(Oriel 76175_71580) and a dual-channel power meter (Newport 2931_C).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

What is claimed is:
 1. A method of producing an electrode of adye-sensitized solar cell, the method comprising: dispersing a pluralityof semiconductor nanoparticles on a transparent electrically conductivesubstrate; dispersing a plurality of semiconductor nanofibers on thesemiconductor nanoparticle layer; adsorbing onto all sides of thesemiconductor nanofibers a first light absorption material, therebysensitizing the semiconductor nanofibers, wherein the light absorptionmaterial has a first light absorption bandwidth; and depositing a secondlight absorption material in contact with and forming respective shellson the respective semiconductor nanofibers on which the first lightabsorption material is adsorbed, wherein the second light absorptionmaterial has a second light absorption bandwidth complementary to thefirst light absorption bandwidth.
 2. The method of claim 1, wherein thesemiconductor nanofibers have a porous structure.
 3. The method of claim1, including depositing the second light absorption material on thefirst light absorption material via a deep penetration technique.
 4. Themethod of claim 3, wherein the deep penetration technique is selectedfrom the group consisting of thermal, chemical, and physical vapordeposition methods.
 5. The method of claim 1, wherein the first lightabsorption material has an energy level that is higher than an energylevel of a conduction band of the semiconductor nanofibers, and thesecond light absorption material has an energy level that is higher thanthe energy level of the first light absorption material.
 6. A method ofproducing an electrode of a dye-sensitized solar cell, the methodcomprising: dispersing a plurality of semiconductor nanoparticles on atransparent electrically conductive substrate; dispersing a plurality ofsemiconductor nanofibers on the semiconductor nanoparticle layer;adsorbing onto all sides of the semiconductor nanofibers a first lightabsorption material, thereby sensitizing the semiconductor nanofibers,wherein the light absorption material has a first light absorptionbandwidth and an energy level higher than that of a conduction band ofthe semiconductor nanofibers; and depositing additional light absorptionmaterials, successively, and forming a shell structure, with eachsuccessively deposited light absorption material being deposited on apreviously deposited light absorption material, each successivelydeposited light absorption material having a higher energy level thanthe light absorption material previously deposited.
 7. The method ofclaim 6, wherein each of the successively deposited light absorptionmaterials has a corresponding light absorption bandwidth that iscomplementary to a light absorption bandwidth of the previouslydeposited light absorption material materials.